Defensive symbiont diversity contributes to parasitoid coexistence

Following its unintentional introduction, the pea aphid, Acyrthosiphon pisum, became a major pest of leguminous crops across North America. While several resident parasitoids shifted to attacking pea aphids, none achieved effective control. The subsequent introduction of Aphidius ervi successfully suppressed pea aphid populations but displaced all parasitoids except the native Praon pequodorum. These species now stably coexist, though the underlying mechanisms remain unclear. Pea aphids resist parasitoids primarily through defensive endosymbionts. Most symbiont strains protect against the co-evolved A. ervi, but newly discovered strains specifically target P. pequodorum. We hypothesized that symbiont diversity creates refuge hosts for each parasitoid, enabling P. pequodorum to persist despite A. ervi’s superior host location abilities. We tested this hypothesis through population cage experiments with four aphid treatments: populations with symbionts that protected against 1) A. ervi only, 2) P. pequodorum only 3) both parasitoids, or 4) neither parasitoid, with three replicates per treatment. Equal numbers of both parasitoid species were introduced to each cage and allowed to forage briefly before removal. After ten days, we counted parasitoid mummies and reset cages for subsequent rounds, adjusting parasitoid ratios based on reproductive success. Generalized linear mixed model analysis revealed that in cages with single symbiont types, the targeted parasitoid was competitively excluded within three rounds. However, in cages with both symbiont types, both parasitoid species coexisted, demonstrating that symbiont diversity maintains parasitoid diversity. Since parasitoid diversity stabilizes pest suppression, understanding coexistence mechanisms has important implications for biological control.